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Early yesterday, BART management offered the unions a wage increase of 18.5 percent over four years, compared with the transit district's original proposal of 11 percent over four years.

The unions responded at 5 p.m. yesterday with a proposed salary hike of 20 percent over three years, down from the 26.5 they initially sought.

Both sides said the length of the contract was a major sticking point. BART officials say a longer contract allows them to plan ahead, but the unions say it could box them in. They expected negotiations to continue into the night and beyond.

The dispute principally concerns wages, as well as pension contributions and work rules, including outstanding grievances. The two sides have reached agreement on health benefits, and management has conceded that it would maintain benefits without increasing employees' costs.

If 2,800 BART employees strike Wednesday morning, it is expected to create gridlock throughout the Bay Area, as was the case during the six-day strike in 1997.

Yesterday, regional air-quality regulators said the traffic chaos would also substantially increase air pollution.

If as few as half of BART's 330,000 daily riders decided to use their cars, 7 to 14 tons of ozone components, in addition to another 28 to 56 tons of carbon monoxide, would be released into the air, according to the Bay Area Air Quality Management District.

BART directors called on the unions yesterday morning to accept their latest proposal by the end of the day, so BART riders "do not have to face anxiety this weekend over what will happen when they come back from Labor Day, " said board member Dan Richard.

Transit district directors and management said their offer yesterday would keep salaries ahead of inflation in the Bay Area, but union leaders -- using different figures -- disagreed.